1. Technical Field
This invention relates to an image scanner that scans images on a document and outputs them as bilevel images for electronic storage, transmission and printing of documents.
2. Description of the Prior Art
Among the advantages of image scanners are that they enable document image storage, transmission, printing and other such processing to be carried out quickly and at low cost. Such scanners are therefore used to convert the patterns making up such images to bilevel image signals corresponding to black and white levels. To ensure that low contrast patterns are also binarized rather than being erased during the conversion process, it is necessary to set the binarization threshold as closely as possible to the background. However, variations in the background brightness caused by paper lint and the like result in the inclusion of granular noise in the bilevel image. This background noise degrades the quality of the bilevel image and also lowers image compression efficiency.
In conventional image scanners, background noise is suppressed either by making all patterns uniformly thin or by the circle method wherein the pixels of interest are compared with surrounding patterns, and if all of the surrounding patterns are the same, the pixels of interest are combined with the surrounding patterns. Making the patterns uniformly thin can, however, result in the erasure of the pattern of interest. With the circle method, an increase in the size of the background noise elements can cause the number of combinations of circles to be compared to increase exponentially, making it difficult to carry out the necessary processing and therefore preventing effective suppression of the background noise. Thus, with conventional image scanning systems it is difficult to binarize the images without erasing low contrast patterns, and it is also difficult to produce bilevel images with low background noise. Accordingly, it will be appreciated that it would be highly desirable to binarize the images without erasing low contrast patterns and to produce bilevel images with low background noise.